For radio communication terminals such as cellular phones and for radio transmitters such as mobile communication base stations, there is demand for amplifiers with excellent power saving characteristics and little distortion. In general, in order to satisfy distortion properties, the transmission RF power amplifier in a transmitter is used at an output level having good linearity and sufficient back-off from saturation power. As a result, however, the transmission RF power amplifier is used in a state of poor power efficiency, thereby increasing power consumption and leading to an increase in size of the transmitter. A variety of techniques have therefore been proposed for improving the efficiency of a transmission RF power amplifier, such as Doherty amplifiers and Envelope Tracking amplifiers (for example, see Patent Literature 1-3 and Non-Patent Literature 1 and 2).
FIG. 7 illustrates the circuit configuration of an analog ET amplifier, and FIG. 8 illustrates the circuit configuration of a digital ET amplifier. With ET, an input signal is divided in two, with one input signal being input into a detection circuit so that only the amplitude signal component (envelope signal) is extracted. An amplifier that controls the power supply voltage of the transmission RF power amplifier (referred to below as “envelope amplifier”) controls the power supply voltage of the transmission RF power amplifier with a voltage proportional to the envelope signal. Via a limiting circuit, the other input signal becomes a high-frequency signal with only the phase signal component, and the transmission RF power amplifier can output a transmission RF signal that includes the original envelope by amplifying the high-frequency signal at the power supply voltage provided by the envelope amplifier.
The definition for the efficiency of the transmission RF power amplifier is generally given by Equation 1. In this equation, Vout represents the output voltage, Iout represents the output current, Vdd represents the supply voltage (power supply voltage), and Idd represents the supply current.efficiency=Vout×Iout/Vdd×Idd  (1)
Equation 1 above demonstrates that if Vdd, which is the power supply voltage, is varied to be lower than a fixed power supply voltage, the efficiency can be improved as compared to the fixed power supply voltage. In particular, as described above an ET amplifier can improve amplifier efficiency since the power supply voltage supplied to the transmission RF power amplifier from the envelope amplifier changes in response to the envelope signal.
FIG. 9 illustrates the circuit configuration of an ET envelope amplifier. Note that in FIG. 9, the transmission RF power amplifier is illustrated with an equivalent load Rload. The input signal into the envelope amplifier is first added to a voltage follower amplifier, and current flows to the load Rload through the load Rsense. Meanwhile, a comparator (hysteresis width h) operates due to the voltage generated at either end of the load Rsense, and when a MOS-FET turns ON, current flows to the load Rload from a coil L. As a result, the voltage at the load Rload side of the load Rsense rises, so that the comparator inverts and the MOS-FET turns OFF. Such a circuit transmits of its own accord and generates a PWM signal.
In an ET envelope amplifier, a semiconductor device such as a MOS-FET is used as a switch as described above. For example, according to Non-patent Literature 1, when a 20 MHz OFDM signal is assumed, a switching frequency of 100 MHz is necessary in order to reproduce a detailed envelope.